The obtaining of optical elements such as glass lenses by precision mold pressing is known. Optically functional surfaces molded by press molding afford desired optical performance without requiring mechanical processing such as polishing.
Patent Document 1 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 08-277125, the entire contents of which are hereby incorporated herein by reference) describes a method of forming a coating of a group IIIa metal oxide on the surfaces of a glass lens molding material on which functional surfaces are to be molded. The prevention of fusion of the molding-use material and outer surface of the mold is disclosed.
Patent Document 2 (Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-226825, the entire contents of which are hereby incorporated herein by reference) describes a method of press molding a glass product in a softened state using a mold comprised of glass having a glass transition temperature higher than the pressing temperature, in which a fusion-preventing layer is inserted between the molded glass product having a shape constituting the foundation of the finished shape of the lens and the mold.
Based on the method described in Patent Document 1, a metal oxide coating of a group IIIa element in the Periodic Table of the Elements, such as yttrium oxide, cerium oxide, or lanthanum oxide, is formed in advance on surfaces of the glass material on which functional surfaces are to be molded and press molding is conducted. SiO2—B2O3—La2O3 glass, P2O5—TiO2—Nb2O5 glass, and the like are described as the glass material for molding.
However, the physical properties of the above metal oxide coating differ greatly from those of the glass material. Since softening is not identical over the temperature range at which the glass material softens, when the glass material is pressed by the pressing surfaces of the pressing mold to deform during press molding, it cannot extend by flexibly following the deformation. In addition, during the subsequent cooling process, the shrinkage rate (thermal expansion rate) differs from that of the glass material, ending up producing breaks in the coating. As a result, it becomes impossible to prevent the glass material from being partially pressed directly against the pressing surface, creating a problem in that fusion cannot be prevented.
The method described in Patent Document 2 prevents fusion between the glass and the mold occurring when molding glass lenses with a mold made of glass by inserting a fusion-preventing layer between them.
Based on investigation, the present inventors discovered that, distinct from the fusion occurring between glass and mold when press molding a glass material with a mold made of glass, when the glass material contains specific components, and even when a mold made of a material other than glass is employed, an interface reaction occurs due to the components, making it difficult to obtain optical components of adequate performance.
When the above undesirable interface reaction occurs during the molding of an optical glass element by precision mold pressing, it causes scratch-like reaction marks on the glass surface, clouding, and wear and tear of the pressing mold, precluding the satisfactory molding of a glass element such as a lens affording good optical performance and external appearance.
Based on the application of the optical element obtained by molding, various optical constants and physical and chemical properties may be desirable in a glass material employed in precision mold pressing. In particular, optical glasses of high refraction (for example, an nd of 1.7 or higher) and high dispersion (v(nu)d of 35 or lower), which are useful in small image pickup apparatuses and the like, and optical glasses of high refraction (same as above) and low dispersion (such as a v(nu)d of 65 or higher), are valued in such image pickup apparatuses and the like, so the need is great. The present inventors have developed a highly refractive glass capable of filling this requirement.
The use of W, Ti, Bi, and Nb as glass components is advantageous for achieving a high refractive index. However, since these components can assume a number of valences when present as glass components, they tend to produce an oxidation reduction reaction and are readily reduced by the atmosphere and temperature in the course of press molding, for example. In a glass material containing such typical highly reactive components, various interface reactions take place in the deformation process while being pressed by the pressing mold at the pressing temperature. Fusion to the pressing mold, clouding of the surface of the glass molded product, and residual scratch-like reaction marks have been discovered. The pressing surface is roughened by fusion, the transferal of this roughness produces irregularities in the surface of the molded product, and a tendency toward clouding has been observed.
Imparting abnormal dispersion to an optical element is advantageous to correct chromatic aberration in optical apparatuses. To this end, fluorine-containing optical glass is usefully employed. However, when a glass material comprised of fluorine-containing optical glass is press molded, contamination of mold surfaces due to volatilization of the fluorine on the pressing surface and clouding of the molded product tend to occur. Further, when a metal is incorporated into the pressing surface (often when a metal mold release film is provided on a pressing surface), there is a strong reaction with the fluorine, and a tendency for scratch-like reaction marks to occur on the lens surface has been observed.
Accordingly, the present invention has for its object to permit the stable production of optical elements affording adequate optical performance by using optical glass containing these components and inhibiting undesirable interface reactions during press molding.